1. Field of the Invention
The invention relates generally to the field of roller cone (“rock”) bits used to drill wellbores through earth formations. More specifically, the invention is related to structures for cutting elements (“inserts”) used in roller cone bits having a single roller cone.
2. Background Art
Roller cone bits are one type of drill bit used to drill wellbores through earth formations. Roller cone bits include a bit body adapted to be coupled to a drilling tool assembly or “drill string” which rotates the bit as it is pressed axially into the formations being drilled. The bit body includes one or more legs, each having thereon a bearing journal. The most commonly used types of roller cone drill bits include three such legs and bearing journals. A roller cone is rotatably mounted to the bearing journal. During drilling, the roller cones rotate about the respective journals while the bit is rotated. The roller cones include a number of cutting elements, which may be press fit inserts made from tungsten carbide and other materials, or may be milled steel teeth. The cutting elements engage the formation in a combination of crushing, gouging and scraping or shearing action which removes small segments of the formation being drilled. The inserts on a cone of a three-cone bit are generally classified as inner-row insert and gage-row inserts. Inner row inserts engage the bore hole bottom, but not the well bore wall. Gage-row inserts engage the well bore wall and sometimes a small outer ring portion of the bore hole bottom. The direction of motion of inserts engaging the rock on a two or three-cone bit is generally in one direction or a very small limited range of direction, i.e., 10 degrees or less.
One particular type of roller cone drill bit includes only one leg, bearing journal and roller cone rotatably attached thereto. The drilled hole and the longitudinal axis of this type of bit are generally concentric. This type of drill bit has generally been preferred for drilling applications when the diameter of the hole being drilled is small (less than about 4 to 6 inches [10 to 15 cm]) because the bearing structure can be larger relative to the diameter of the drilled hole when the bit only has one concentric roller cone. This is in contrast to the typical three-cone rock bit, in which each journal must be smaller relative to the drilled hole diameter.
An important performance aspect of any drill bit is its ability to drill a wellbore having the full nominal diameter of the drill bit from the time the bit is first used to the time the cutting elements are worn to the point that the bit must be replaced. This a particular problem for single cone bits because of the motion (trajectory) of the cutting elements as they drill the wellbore. Essentially all but a few centrally positioned cutting elements on the cone eventually engage the wellbore wall at the gage diameter. The inserts on a single cone bit go through large changes in their direction of motion, typically anywhere from 180 to 360 degrees. Such changes require special consideration in design. The inserts on a single cone bit undergo as much as an order of magnitude more shear than do the inserts on a conventional two or three cone bit. Such amounts of shear become apparent when looking at the bottom hole patterns of each type of bit. A single cone bit creates multiple grooves laid out in hemispherically-projected hypotrochoids, a configuration similar to ink paths generated by drawing instruments in a toy sold under the trade mark SPIROGRAPH by Tonka Corp., Minnetonka, Minn. 55343. A two or three cone bit, in contrast, generates a series of individual craters or indentations. Shearing rock to fail it will typically cause more wear on an insert than indenting an insert to compressively fail rock. Therefore, the inserts on a single cone bit wear faster than the inserts on a two or three cone bit. As the cutting elements on a single cone bit wear, therefore, the drilled hole diameter reduces correspondingly.
One way to maintain full drilled diameter in a single cone bit is to include fixed cutters on the bit body. The fixed cutters may be tungsten carbide inserts. Typically, the fixed cutters will be affixed to the bit body at a position axially above the roller cone on the bit. A single cone bit known in the art which includes the foregoing features is described in U.S. Pat. No. 6,119,797 issued to Hong et al. The bit shown therein includes special inserts in an “intermittent contact zone” on the roller cone, and both active and passive gage protection inserts or buttons on the bit body axially above the roller cone.
While the bit described in the Hong et al. '797 patent is effective in maintaining full diameter of the drilled hole, using fixed cutters as described increases the “gage length” of the drill bit. This may lessen the ability of such a bit to be used in directional drilling applications. Directional drilling includes drilling the wellbore along a selected trajectory, typically other than vertical. Having fixed cutters and/or gage pads on the bit body also increases the torque required to turn the bit, which is not desirable, and in some cases limits the rotary speed that the bit can be turned, leading to reduced drilling rates.
It is therefore desirable to have a single cone rock bit which can better maintain full gage diameter during its useful life, while remaining useful in directional drilling applications.